The present invention relates to a numerical control apparatus for controlling machining of non-circular workpieces (hereinafter merely referred to as a workpiece) such as a cam and the like.
Conventionally, a process for grinding the workpiece such as the cam by controlling a feed of grinding wheel the direction of which is perpendicular to a main spindle, with the numerical control apparatus in synchronism with the rotation of the main spindle is known. It is necessary to provide profile data to the numerical control apparatus for synchronizing control of the grinding wheel feed. The profile data provides the feed amount of the grinding wheel per a unit rotational angle of the main spindle so as to reciprocate the grinding wheel, that is to give it the profile creating motion along the finished shape of the workpiece.
The profile data is obtained from lift data of the non-circular workpiece and the grinding wheel diameter.
While, though the non-circular workpiece such as the cam is used to change the circular motion into linear motion, displacement characteristics of a tappet in contact with the surface of the cam relative to the rotational angle of the cam is required from the performance of machines using the cam. For example, in the case of the cam-shaft of an internal combustion engine, in order to improve acceleration and high speed performance of a motor vehicle, the relationship between a crank angle and the tappet displacement is determined from the functional aspect.
As such, when designing the cam shape, since the relationship between the rotational angle and the linear motion displacement for realizing the desired motion is given in sequence of points from the functional aspect of the cam, the cam shape for realizing the motion accurately does not necessarily vary smoothly.
Though the non-circular workpiece is machined by controlling the rotating position of the main spindle and the feeding position of the grinding wheel in synchronism, since follow-up delay and errors are occurred in a servo-control mechanism due to the inertia of movable bodies such as a wheel head and the clearance of a feed screw, it is impossible to machine the workpiece exactly to the finished shape required from the operating characteristics of the cam. The follow-up errors also become larger as the rotating speed of the main spindle increases.
Accordingly, in order to machine the workpiece as close as possible to the required finished shape, machining must be performed extremely slowly.
The more are the higher order frequency components included in the required finished shape of the cam, the more are the high frequency vibration components included in the servo-control mechanism, deteriorating the machining accuracy.